Opiate abuse potentiates the neuropathogenesis of HIV by synergistically increasing dendritic pathology (varicosity formation, beading, fragmentation, pruning), while promoting additive dendritic spine losses (plasticity). Behavioral deficits in spatial and non-spatial memory tasks are accompanied by synaptic losses and dendritic pathology preceding neuron death, suggesting that neuronal injury and reduced synaptic connectivity underlie the ability of opioids to aggravate HIV-associated neurological disorders (HAND). We have found that phenotypically distinct subpopulations of hippocampal CA1 interneurons appear to be highly sensitive to Tat opiates, and disruptions to these interneuron subpopulations may contribute, in part, to pyramidal cell dysfunction/injury. These findings represent a fundamental shift in our understanding of opioid drug action and propel the grant in novel directions. We hypothesize that opiates and HIV-induced hippocampal behavioral dysfunction is caused by disruptions to synaptic function and organization in vulnerable neuronal subpopulations that disrupt specific neural networks within the hippocampus. Aim 1 will characterize the neurophysiologic events underlying opioid and HIV-dependent neuronal dysfunction and injury in hippocampal CA1 pyramidal cells in whole-cell, patch-clamp recordings of CA1 pyramidal cells. Alterations in long-term potentiation and depression will be explored, as will deficits in subthreshold postsynaptic potentials in response to opiates and Tat. During patch-clamp recordings, neurons will be biocytin-filled and subsequently analyzed via 3D-reconstruction for dendritic pathology and spine density. Aim 2 will determine how vulnerable subsets of MOR-expressing CA1 interneurons exacerbate opiate and HIV-1-induced hippocampal dysfunction, neuronal injury, and disrupt network function. Tat tg mice will be crossed with MORfloxed;VGAT-Cre mice, which lack MOR+ CA1 interneurons. We will also determine how Tat and opiates affect synaptic processing and network function in CA1 by examining the integration of synaptic inputs by imaging genetically encoded-voltage indicators (GEVIs) selectively expressed in CA1 pyramidal neurons. Aim 3 will identify the neurophysiologic mechanisms underlying opiate and infectious HIV-1/HIV protein-induced neuronal dysfunction and injury in human hippocampal neurons. Opiate and Tat-induced neurophysiological and structural deficits will be correlated with deficits seen in Tat mice assessed in Aims 1 and 2. Our long-term goal is to define the mechanisms by which opiate drug abuse exacerbates neurodegenerative and functional defects, and to identify key underlying events that could be targeted therapeutically.